Light-phase-noise error reducer

ABSTRACT

A resonator gyroscope comprises a reference laser generator to produce a reference light; a first slave light source to produce a first slave light locked to the reference light; a second slave light source to produce a second slave light locked to the reference light; a first optical filter cavity coupled to at least one of the first and second slave light sources to filter out high-frequency fluctuations in the respective first and second slave lights; a resonator coupled to said first and second light sources, the resonator having first and second counter-propagating directions and resonance tracking electronics coupled to the resonator to generate a first beat frequency, a second beat frequency, and a third beat frequency; wherein the rotational rate of the resonator gyroscope is a function of the first, second and third beat frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/285,990, filed on Dec. 13, 2009 and entitled “LIGHT-PHASE-NOISEERROR REDUCER”, which is referred to herein as the '990 application.

This application is related to co-pending U.S. patent application Ser.No. 12/636,741 (attorney docket number H0023877-5704) entitled “SYSTEMAND METHOD FOR REDUCING LASER PHASE NOISE IN A RESONATOR FIBER OPTICGYROSCOPE” filed on Dec. 13, 2009 and which is referred to herein as the'877 application. The '877 application is hereby incorporated herein byreference.

BACKGROUND

Gyroscopes (also referred to herein as gyros) have been used to measurerotation rates or changes in angular velocity about an axis of rotation.A basic conventional fiber optic gyro (FOG) includes a light source, abeam generating device, and a coil of optical fiber coupled to the beamgenerating device that encircles an area. The beam generating devicetransmits light beams into the coil that propagate in a clockwise (CW)direction and a counter-clockwise (CCW) direction along the core of theoptical fiber. Many FOGs utilize glass-based optical fibers that conductlight along a solid glass core of the fiber. The two counter-propagating(e.g., CW and CCW) beams experience different pathlengths whilepropagating around a rotating closed optical path, and the difference inthe two pathlengths is proportional to the rotational rate that isnormal to the enclosed area.

In a resonator fiber optic gyro (RFOG), the counter-propagating lightbeams are typically monochromatic (e.g., in a single frequency) andcirculate through multiple turns of the fiber optic coil and formultiple passes through the coil using a device, such as a fibercoupler, that redirects light that has passed through the coil back intothe coil again (i.e., circulates the light). The beam generating devicemodulates and/or shifts the frequencies of each of thecounter-propagating light beams so that the resonance frequencies of theresonant coil may be observed. The resonance frequencies for each of theCW and CCW paths through the coil are based on a constructiveinterference condition such that all light-waves having traversed thecoil a different number of times interfere constructively at any pointin the coil. As a result of this constructive interference, an opticalwave having a wavelength k is referred to as “on resonance” when theround trip resonator pathlength is equal to an integral number ofwavelengths. A rotation about the axis of the coil produces a differentpathlength for clockwise and counterclockwise propagation, thusproducing a shift between the respective resonance frequencies of theresonator, and the frequency difference, such as may be measured bytuning the CW beam and CCW beam frequencies to match the resonancefrequency shift of the closed optical path due to rotation, indicatesthe rotation rate.

SUMMARY

In one embodiment, a resonator gyroscope is provided. The resonatorgyroscope comprises a reference laser generator to produce a referencelight; a first slave light source to produce a first slave light lockedto the reference light; a second slave light source to produce a secondslave light locked to the reference light; a first optical filter cavitycoupled to at least one of the first and second slave light sources tofilter out high-frequency fluctuations in the respective first andsecond slave lights; a resonator coupled to said first and second lightsources, the resonator having first and second counter-propagatingdirections and resonance tracking electronics coupled to the resonatorto generate a first beat frequency, a second beat frequency, and a thirdbeat frequency; wherein the rotational rate of the resonator gyroscopeis a function of the first, second and third beat frequencies.

DRAWINGS

Understanding that the drawings depict only exemplary embodiments andare not therefore to be considered limiting in scope, the exemplaryembodiments will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a block diagram of one embodiment of a resonator gyroscope.

FIG. 2 is a block diagram of another embodiment of a resonatorgyroscope.

FIG. 3 is a graph of an exemplary clockwise intensity waveform and anexemplary counter-clockwise intensity waveform.

FIG. 4 is a block diagram of one embodiment of a multi-frequency lasersource assembly.

FIG. 5 is a graph of an exemplary resonance curve of a referenceresonator.

FIG. 6 is a block diagram of one embodiment of a ring resonator opticalfilter cavity.

FIG. 7 is a block diagram of one embodiment of a MFLS opticssubassembly.

FIG. 8 is a block diagram of another embodiment of a ring resonatoroptical filter cavity.

FIG. 9 is a block diagram of one embodiment of resonance trackingelectronics.

FIG. 10 is a block diagram of one embodiment of a reference lasergenerator.

FIG. 11 is a block diagram of another embodiment of a MFLS opticssubassembly.

FIG. 12 is a flow chart depicting one embodiment of a method ofdetermining a rotational rate of a resonator gyroscope.

FIG. 13 shows an exemplary light frequency transfer function of anexemplary optical filter cavity.

FIG. 14 shows an exemplary Bode plot of loop gain for an exemplaryfeedback loop.

FIG. 15 is a block diagram of an exemplary embodiment of an opticalfilter resonator.

FIG. 16 is a block diagram of an exemplary embodiment of an opticalfrequency discriminator resonator.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize specific features relevantto the exemplary embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments. However, it is tobe understood that other embodiments may be utilized and that logical,mechanical, and electrical changes may be made. Furthermore, the methodpresented in the drawing figures and the specification is not to beconstrued as limiting the order in which the individual steps may beperformed. The following detailed description is, therefore, not to betaken in a limiting sense.

As used herein, the terms “light source” and “laser” areinterchangeable. Similarly, as used herein, the terms “laser beam” and“light” are interchangeable. Additionally, the terms “beam splitter” and“optical coupler” can be used interchangeably herein when the beamsplitter or optical coupler functions to separate a portion of a laserbeam into a different path. The terms “optical filter cavity” and“optical filter resonator” are also used interchangeably herein.

FIG. 1 is a block diagram of one embodiment of a resonator gyro 10. Theresonator gyro 10 comprises tunable light sources 12, 14 (e.g., tunablelasers) that synthesize light beams, respectively, and a resonator 25circulating light beams in counter-propagating directions. The resonator25 includes a recirculator 40 and a resonator coil 24. The recirculator40 introduces a portion of the light beams from the tunable lightsources 12, 14 into the resonator coil 24. For example, the recirculator40 is comprised of a plurality of mirrors and a plurality of polarizersin some embodiments.

The resonator gyro 10 also includes photodetectors 16, 18 that samplelight circulating in the resonator 25, and resonance detectors 34, 38coupled to the photodetectors 18, 16, respectively, that detect thecenters of resonance dips for each of the counter-propagating directionsof the resonator 25. The resonator gyro 10 also includes servos 36, 35having an input coupled to the resonance detectors 34, 38, respectively,and an output coupled to the tunable light sources 14, 22, respectively.These components of the resonator gyro 10 thus form resonance trackingloops 30, 32 for each counter-propagating direction (e.g., clockwise(CW) and counter-clockwise (CCW)).

The light beam produced by the first tunable laser 12 (e.g., CCW laser)is tuned to a frequency f0, and the light beam produced by the secondtunable laser 14 (e.g., CW laser) is tuned to a frequency f0+Δf. Therelative frequency drift and jitter between the two laser frequencies ispreferably substantially minimized to a level that minimizes or does notaffect the accuracy and stability of the frequency shift, and thusrotational rate, measurement. This can be accomplished by locking thelaser frequencies to the resonance frequencies with servos 36, 35 ofresonance tracking loops 30 and 32 having sufficient loop gain withinthe rotation measurement frequency band. Sufficient loop gain isobtained by selecting a modulation frequency that is substantiallygreater than the required unity gain frequency for the resonancetracking loops 30, 32. For example, the modulation frequency is at leastabout four times the unity gain frequency for the resonance trackingloops 30, 32 in some embodiments. Additionally, this can be accomplishedby the laser frequency stabilization techniques described below. Each ofthe tunable lasers 12, 14 sinusoidally frequency modulates the lightbeams at the respective frequencies. Additionally, the resonator gyro 10may include additional mirrors 20, 22 and beam splitters 26, 28 fordirecting the propagation of light beams from the tunable lasers 12, 14to the resonator 25 and for directing light from the resonator 25 to thephotodetectors 16, 18.

The resonator 25 comprises the recirculator 40 and an optical fiber coil24 having first and second ends coupled to the recirculator 40. In someembodiments, the optical fiber coil 24 is a hollow core optical fibercoil. The recirculator 40 introduces the modulated light beams (e.g., CWand CCW input light beams) into the optical fiber coil 24 and circulatesa portion of the modulated light beams through the optical fiber coil24. The recirculator 40 reintroduces light emerging from one end of theoptical fiber coil 24 into the other end of the fiber coil 24, thuscausing light to propagate through the fiber coil 24 many times.

After receiving the modulated light beams from the tunable lasers 12,14, the recirculator 40 directs a portion of the two modulated lightbeams in counter-propagating directions (e.g., CW and CCW directions).By application of the Sagnac Effect, the optical gyro 10 senses arotation rate about an axis of the optical gyro 10. The photodetectors18, 16 convert optical signals representing the circulating light beamsto electrical signals, and the resonance detectors 34 and 38 detect theresonance centers of the resonance lineshapes for the CW and CCWcirculating light beams and determine the resonance frequenciesassociated with each of the counter-propagating directions of theresonator 25 based on the frequency shift between the detected resonancecenters. The frequency shift is used to determine the rotation rate ofthe optical gyro 10. For example, the first light beam (e.g., a CW beam)has an unshifted laser frequency f0 and is introduced into the resonator25. For rotation sensing, the frequency f0 of the CW beam is tuned(e.g., by tuning the frequency of the laser 12) to the resonancefrequency of the resonator 25 in the CW direction. The second light beam(e.g., a CCW beam) is tuned the frequency f0+Δf to align the CCW beamfrequency with a resonance center relative to the resonance frequency ofthe resonator 25 in the CCW direction.

To measure the resonance center-frequencies in either the CW directionor CCW direction, a standard synchronous detection technique is used.Each input light beam is sinusoidally phase-modulated, and thereforefrequency modulated at frequencies fm and fn, respectively, to dithereach input beam frequency across a resonance lineshape as measured bythe photodetectors 18, 16. For example, additional circuitry coupled tothe photodetectors 18, 16 may demodulate the output of thephotodetectors 18, 16 at the frequencies fm and fn, respectively, tomeasure resonance centers indicated by the light outputs of the CW andCCW beams. At a line center of the resonance lineshape, or the resonancecenter, the optical sensor 16 detects a null output at the fundamentalfrequencies fm and fn, respectively. When the input beam frequency(e.g., f0+Δf or f0) is off-resonance, an error signal at frequencies fmand fn, respectively, is sensed by the photodetector and used to tunethe respective beam frequency to the respective resonance frequency ofthe resonator 25. The frequency of the CW beam is tuned by changing thefrequency, f0, of the laser 12 and the frequency of the CCW beam isadjusted via a feedback loop that changes the frequency shift, Δf, ofthe second laser 14 so that f0+Δf matches the CCW resonance frequency ofthe resonator 25.

In the absence of rotation, the round-trip path-lengths of the CW andCCW beams inside the resonator 25 in the CW and CCW direction,respectively, are substantially equal. Thus, Δf is tuned to zero by thesecond laser 14. In the presence of rotation, the round-trippath-lengths differ between the CW and the CCW directions producing aresonance frequency difference between the two directions that isproportional to the rotation rate. By tuning the frequency f0 to trackthe CW resonance and the frequency Δf to track the CCW resonance center,the rotation rate is determined.

In this example, the CW and CCW beams propagate through a hollow core,band-gap, optical fiber having an extremely low bend loss, and the coil24 has a large number of turns about a substantially small area toachieve a compact gyro. For example, the coil 24 may have from about20-40 turns of the optical fiber about a one centimeter diameter. Thehollow core optical fiber is typically glass-based with a plastic outerjacket and a hollow inner core. In the hollow core optical fiber, lightinjected from the recirculator 40 traverses mostly through free space(e.g., air or a vacuum) along the core, and only about a few percent orless of the optical energy of light is contained in the glass walls ofthe fiber surrounding the hollow core. Because a large majority of thelight energy traverses through free space along the hollow core ofoptical fiber, the transition between the recirculator 40 and the hollowcore optical fiber has a near-perfect index matching, and a highreflectivity laser mirror with low loss and attractive polarizationproperties may be used for the recirculator 40. The hollow core fiber issuited to significantly attenuate the rotation measurement errorscommonly associated with the properties of the glass medium in the coreof conventional fibers. However, it is to be understood that other typesof optical fiber can be used in other embodiments.

The CW resonance tracking loop 30 locks the CW laser 12 onto a CWresonance frequency of the resonator 25, and the CCW resonance trackingloop 32 locks the CCW laser 14 onto a CCW resonance frequency of theresonator 25. The output of each of the resonance detectors 34, 38 is anerror signal indicating a laser frequency deviation away from theresonance frequency of the resonator 25. The servos 36, 35 tune thelasers 12, 14 to maintain the error signal, and thus the frequencydeviation, substantially at zero.

FIG. 2 is a block diagram of another embodiment of a resonator gyro 50.The resonator gyro 50 comprises a multi-frequency laser source (MFLS)assembly 52, a Sagnac resonator assembly 54 coupled to an output of theMFLS assembly 52, and a resonance tracking electronics (RTE) system 56having an input coupled to an output of the Sagnac resonator assembly54, a first output coupled to an input of the MFLS assembly 52, and asecond output for providing gyro data of the resonator gyro 50. Using afeedback loop from the Sagnac resonator assembly 54 to the MFLS assembly52 via the RTE system 56, a plurality of lasers in the MFLS assembly 52are tunable to produce modulated light beams that are locked onto theresonance frequency corresponding to a respective propagation directionin the resonator.

The MFLS assembly 52 includes a reference laser generator 210. In oneexemplary embodiment using two slave lasers locked to a stabilizedreference laser from the reference laser generator 210, a first slavelaser produces a CW beam that is tuned to a resonance frequency of theCW direction of the resonator, and a second slave laser produces a CCWbeam that is tuned to a resonance frequency of the CCW direction of theresonator. The resonance frequency of the CCW direction is on adifferent longitudinal resonance mode (e.g., at a resonance frequencythat is at least one longitudinal resonance mode away from the resonancefrequency of the CW direction) than the resonance frequency of the CWdirection. The frequency spacing between adjacent modes is termed thefree spectral range (FSR). To eliminate optical backscatter errors, twolasers are used that are tuned to frequencies at least one longitudinalresonance mode apart. To remove a large bias and associated biasinstabilities (e.g., due to the FSR being part of the measurement), theCCW beam is switched between a CCW resonance frequency that is at leastone longitudinal resonance mode lower than the resonance frequency ofthe CW direction and a CCW resonance frequency that is at least onelongitudinal resonance mode higher than the resonance frequency of theCW direction. By subtracting the beat frequency occurring when theresonance frequency of the CCW direction is one longitudinal resonancemode lower than the resonance frequency of the CW direction from thebeat frequency occurring when the resonance frequency of the CCWdirection is one longitudinal resonance mode higher than the resonancefrequency of the CW direction, an output value is produced that is abouttwo times the frequency difference Δf, as described in more detailbelow.

In another exemplary embodiment using three slave lasers locked to themaster reference laser from the reference laser generator 210, a firstslave laser produces a CW beam that is tuned to a resonance frequency ofthe CW direction of the resonator. A second slave laser produces a CCWbeam that is tuned to a resonance frequency of the CCW direction of theresonator that is at least one longitudinal resonance mode lower thanthe resonance frequency of the CW direction, and a third slave laserproduces another CCW beam that is tuned to a resonance frequency of theCCW direction of the resonator that is at least one longitudinalresonance mode higher than the resonance frequency of the CW direction.By subtracting the beat frequencies, as described below, an output valueis produced that is about two times the frequency difference Δf.Although two slave laser and three slave laser configurations of theresonator gyro 50 are described, additional lasers may be incorporatedwith the resonator gyro 50.

The MFLS assembly 52 comprises an MFLS 58 and an MFLS optics subassembly60 coupled to an output of the MFLS 58 that routes the modulated lightbeams to the Sagnac resonator assembly 54 and provides an opticalfeedback of the modulated light beams to the MFLS. The MFLS opticssubassembly 60 includes, but is not necessarily limited to, the two ormore slave lasers for generating the modulated light beams in responseto control signals from the RTE system 56 and the master reference laserfrom the reference laser generator 210. The Sagnac resonator assembly 54comprises resonator coupling optics 62 (e.g., the recirculator 40 andphotodetectors 16, 18 shown in FIG. 1) having an input coupled to theMFLS optics subassembly 60 and a resonator fiber coil 64 coupled to afirst output of the resonator coupling optics 62. The resonator fibercoil 64 is implemented as a hollow core fiber coil, such as fiber coil24 shown in FIG. 1, and circulates a portion of the modulated lightbeams in counter-propagating directions. The resonator coupling optics62 provides electrical signals of resonator optical outputs (e.g.,intensity measurements of the CW and CCW circulating beams) to the RTEsystem 56 via a second output.

The RTE system 56 comprises a resonance tracking circuit 66 having aninput coupled to the second output of the resonator coupling optics 62and has an output coupled to an input of the MFLS 58. The RTE system 56may comprise additional components such as analog-to-digital (A/D)converters and digital-to-analog converters (DACs) for processingsignals received from the resonator coupling optics 62 and transmittedto the MFLS 58. The RTE system 56 produces laser frequency controlsignals for the slave lasers in the MFLS optics subassembly 60 andapplies a constant offset to the light beams generated by at least oneof the slave lasers. For example, in some embodiments, the constantoffset is based on a longitudinal resonance mode between one resonancecenter and another resonance center, as detected by the photodetectorsin the resonator coupling optics 62 of a corresponding light beam.

FIG. 3 is a graph of an exemplary clockwise intensity waveform 68 and anexemplary counter-clockwise intensity waveform 70 useful inunderstanding the resonator gyro 50. Referring to FIGS. 2 and 3, in anexemplary embodiment, when the CW beam is tuned to the resonancefrequency of the CW direction of the resonator coil 64, the CW intensitywaveform 68 is observed having resonance dips 72, 74 occurring atdifferent longitudinal resonance modes. Similarly, when the CCW beam istuned to the resonance frequency of the CCW direction of the resonatorcoil 64, the CCW intensity waveform 70 is observed having resonance dips76, 78 occurring at different longitudinal resonance modes. The centersof these resonance dips 72, 74, 76, 78 indicate resonance frequencies atdifferent longitudinal resonance modes.

For an RFOG system with four lasers, one being a reference laser (ormaster laser) and the other three being resonance tracking lasers (orslave lasers), as described below, the slave lasers are phase locked tothe reference lasers with independent controllable frequency offsets foreach slave laser. The frequency (fr) of the reference laser is set suchthat the beat frequencies between the slave lasers and the referencelaser are within normal operating limits of the gyro electronics whilethe slave lasers are locked to the resonator. In particular, the firstslave laser is tuned to a CW resonance frequency f_(CW) or f₁, thesecond slave laser is tuned to a first CCW resonance frequency,f_(CCW,1,) or f₂, that is one longitudinal mode lower than the CWresonance frequency F_(CW) at zero rotation rate of the resonator gyro50, and the third slave laser is tuned to a second CCW resonancefrequency, f_(CCW,2) or f₃, that is one longitudinal mode higher thanthe CW resonance frequency F_(CW) at zero rotation rate of the resonatorgyro 50.

In one example, the reference frequency, f_(r), is set to be higher thanthe slave frequencies. In this example the slave beat frequencies forslave 1, 2 and 3 respectively are: Δf₁=f_(r)−f₁, Δf₂=f_(r)−f₂, andΔf₃=f_(r)−f₃. The RTE 56 of FIG. 2 generates the three beat frequenciesusing direct digital synthesizer (DDS) chips. The RTE 56 controls thethree beat frequencies to maintain the slave lasers on their respectiveresonance frequencies. The gyro data Δf₁, Δf₂ and Δf₃ can be output to aprocessor that makes the calculation (Δf₁−Δf₃)−(Δf₂−Δf₁)=2ΔfΩ, where ΔfΩis proportional to rotation rate, Δf₁−Δf₃=f_(FSR)+ΔfΩ, andΔf₂−Δf₁=f_(FSR)−ΔfΩ. Thus, a rotation measurement is obtained withoutFSR and any associated bias and bias instability.

In the two laser exemplary embodiment, the CW beam is locked onto aresonance dip at the resonance frequency f_(CW). The CCW beam is lockedonto a resonance dip at the resonance frequency f_(CCW,1), which is onelongitudinal mode away from the CW resonance (e.g., has one less wavecycle that fits within the resonator ring) at zero rotation rate of theresonator gyro 50. The frequency spacing, f_(FSR) between adjacent modesis termed the free spectral range. Since the FSR depends on the opticalpathlength, which can depend on temperature due to thermal expansion,the resulting bias may be unstable due to temperature variations.

The effects of the FSR are substantially removed by periodicallyswitching the frequency of the CCW beam from the resonance dip that isone longitudinal mode lower (e.g., f_(CCW,1)) than the CW resonance tothe resonance dip that is one longitudinal mode higher (e.g. f_(CCW,2))than the CW resonance. The CW beam is maintained on the resonance dip atthe resonance frequency f_(CW).

FIG. 4 is a block diagram of one embodiment of the multi-frequency lasersource assembly 52 shown in FIG. 2. The MFLS optics subassembly 60comprises a first slave laser 82, second slave laser 84, third slavelaser 80, optical couplers 90, 92, 94, 95, 96, 97, 98, 99, 100, and anoptical fiber 86 coupled to the outputs of each of the lasers 80, 82,and 84, and between the optical couplers 90, 92, 94, 95, 96, 97, 98, 99,100. The optical fiber 86 is implemented as a polarization maintainingsingle mode fiber in this example. The first slave laser 82 produces aCW beam for introduction to the CW input of the resonator coil 64, andthe slave lasers 80, 84 produce CCW beams for introduction to the CCWinput of the resonator coil 64. A portion of each of the CCW beams arecombined by the coupler 100 prior to introduction to the CCW input ofthe resonator coil 64.

The reference laser generator 210 comprises a master laser 402,frequency discriminator 404, photodetector 406, differential amplifier408, servo electronics 412, and laser current drive 413. The masterlaser 402 produces a reference laser beam having a frequency of f0+Δf0,where f0 is the center frequency of the reference laser beam and MDrepresent frequency noise or jitter in the laser beam. The referencelaser beam is passed through the frequency discriminator 404. In theexamples described herein, the frequency discriminator 404 isimplemented as an optical resonator. Hence, the frequency discriminator404 is also referred to herein as a reference resonator. However, it isto be understood that, in other embodiments, other types of frequencydiscriminators can be used.

The frequency to intensity conversion of the reference resonator 404 isincreased by decreasing the linewidth of the optical resonator. Thelinewidth of the optical resonator can be decreased by decreasing theoptical loss within the resonator and increasing the optical pathlengththrough the resonator. The optical pathlength can be greatly increasedif a fiber optic ring resonator is employed, and the resonator fiberlength is made sufficiently long. An optical resonator can provide asufficient frequency to intensity noise conversion factor to overcomethe adverse effects of detection noise. Typical types of detection noiseare laser relative intensity noise (RIN), photon shot noise, and photodetector electronic noise.

Typical optical frequency discriminators have some inherentnon-linearity in their frequency to intensity transfer function. Thenon-linearity will cause frequency fluctuations occurring at very highfrequency to down convert to a lower frequency and result in RIN atlower frequencies. However, if frequency noise at higher frequencies isdown converted to RIN at lower frequencies, then the total intensityvariation at the reference resonator 404 output will not be completelyrepresentative of the laser frequency fluctuations. The down convertednoise will limit how much the laser frequency noise can be reduced.

For an optical resonator the transfer function non-linearity willincrease with a decrease in linewidth. Therefore the down convertednoise will increase with a linewidth decrease. The down conversion noisemechanism imposes a requirement on the optical resonator linewidth thatopposes the requirement set by the detection noise. To minimize theadverse impact of down converted noise, the optical resonator linewidthneeds to be sufficiently large, whereas to minimize the impact ofdetection noise, the linewidth needs to be sufficiently small. Theadverse impact of down converted laser frequency noise can besubstantially reduced by introducing an optical filter cavity before thefrequency discriminator to attenuate laser frequency fluctuations athigh frequency as described in more detail below. The optical filtercavity can be fiber ring resonator very similar to the optical resonatorused as a frequency discriminator. However, the optical filter cavity isoperated in transmission mode.

The reference resonator 404 is configured to have a broad linewidth withhigh linearity at the sides of the curve, as shown in FIG. 5. Theexemplary resonance curve shown in FIG. 5 has a center frequency, fr,and linear portions 501 at the sides of the curve. The linewidth issufficiently broad that both the frequency, f0, and noise, Δf0, of thereference laser beam both fit on the linear portion 501 of the curve. Inparticular, the reference laser generator 210 locks the reference laserbeam to the linear portion 501 of the curve through the resonator.Locking the reference laser beam to the linear portion reduces harmonicsin down converting any jitter above the resonance tracking demodulationfrequency to intensity noise at the resonance tracking demodulationfrequency. In other words, the jitter is converted to intensity noiselinearly. In particular, as the frequency jitters, the output of thereference resonator 404 is input into the photodector 406.

The photodetector 406 sees the intensity fluctuation which is convertedlinearly to an electrical voltage which is output to the differentialamplifier 408. The differential amplifier 408 compares the output of thephotodetector 406 with a stable voltage reference. The differentialamplifier 408 also introduces sufficient gain into the output of thedifferential amplifier 408 so that as the laser jitters one way or theother, the noise is taken out using the servo electronics 412 whichgenerates a control signal based on the output of the differentialamplifier 408. The control signal is input to a current driver 413 whichcauses the master laser 402 to adjust the current of the reference laserbeam which in turn adjust the frequency. After it is locked, thefrequency jitter of the reference laser gets smaller, especially at lowfrequencies. An alternative reference laser generator is shown anddescribed with respect to FIG. 10 below.

Referring to FIG. 4, optical coupler 88 couples a portion of the lightfrom the master laser 402 to the MFLS optics subassembly 60. The opticalcouplers 90, 92, 94, 95, 96, 97, 98, and 99 couple light from the masterlaser 402 with light from the slave lasers 80, 82, and 84 to providefeedback for phase locking the slave lasers 80, 82, and 84 with themaster laser 402. For example, a portion of the reference laser beamproduced by the master laser 402 is mixed with a portion of the CW beamproduced by the first slave laser 80 via the optical couplers 92, 95, 97and 99. A portion of the CCW beam produced by the second slave laser 84is mixed with a portion of the reference laser beam produced by themaster laser 402 via the optical couplers 92, 95, 96, and 98. Similarly,a portion of the CCW beam produced by the third slave laser 80 is mixedwith a portion of the reference laser beam produced by the master laser402 via the optical couplers 92, 94, and 90.

This mixed light is provided to the MFLS 58. The MFLS 58 comprises PhaseLock Loop drive circuits 102, 104, and 106 for each of the lasers 82,84, and 80, respectively. Each PLL beats the light from the respectiveslave laser with light from the master laser 402 and makes therespective slave laser follow the master laser 402. The light from eachslave laser also has low-frequency noise or jitter. However, afterbeating with the reference laser beam, each slave laser is locked to themaster laser 402 which substantially removes the low-frequency jitter.The control signal output from the PLLs 102, 104, 106 controls thefrequency of the slave lasers based on the mixed light received from theMFLS optics subassembly 60 and a corresponding signal from the RTEsystem 56. Although locking the slave lasers to the master laser 402removes low-frequency jitter, any high-frequency jitter in the slavelaser beams is not attenuated by the lock to the reference laser beam.As used herein, high-frequency jitter refers to noise that is too fastto control via servo loops relying on electronic feedback. Similarly, asused herein, low-frequency jitter or fluctuations refers to noise thatis controllable via servo loops relying on electronic feedback.

The MFLS optics subassembly 60 also includes a plurality of opticalfilter cavities 416, 417, and 418. The optical filter cavities 416, 417and 418 substantially remove high-frequency jitter from the laser beamsfrom slave lasers 82, 84, and 80, respectively. In particular, theoptical filter cavities 416, 417, and 418 act as energy storage deviceswhich prevent the transmittance of high-frequency jitter in the laserbeams via a time delay in the optical filter cavities. The time delay ofthe optical filter cavities 416, 417, and 418 is sufficient to roll offand attenuate both laser intensity and frequency noise at frequenciesbeyond the laser linewidth, similar to a low pass filter of anelectronic circuit. The laser beams can be passed through the respectiveoptical filter cavities one or more times to attain sufficientattenuation before being directed to the Sagnac resonator assembly 54.

Since the slave laser beams are intentionally modulated to determineresonance, the optical filter cavities 416, 417, and 418 are configuredso that the intentional modulation of the slave laser beam frequenciesis inside the transmittance curve of the optical filter as described inmore detail below. Thus, the combination of locking the slave laser 82,84, and 80 to the master laser 402 and passing the light from the slavelasers through optical filter cavities 416, 417, and 418 substantiallyremoves both low and high frequency jitter with minimal to noattenuation of the modulation frequency.

In this exemplary embodiment, the optical filter cavities 416, 417, and418 are implemented as ring resonator cavities, such as the exemplaryring resonator cavity 600 shown in FIG. 6. As shown in FIG. 6, theexemplary ring resonator cavity 600 includes mirrors 601, 603, 605, andchannels 607, 609, 611. A laser beam enters the ring resonator cavity600 via one of mirrors 601, 603, 605, such as mirror 601 in thisexample. The laser beam is reflected by mirrors 601, 603, 605 such thatthe laser beam travels around the ring through the channels 607, 609,611. At least a portion of the laser beam passes through another of themirrors, mirror 603 in this example, where it enters an optical fibercoupled to an input of the Sagnac resonator assembly 54. A time-delay isintroduced due to traversing the channels 601, 603, 605 of the ringresonator cavity 600. Thus, high-frequency jitter is not transmittedthrough the ring resonator cavity 600 with the modulated laser beam.

As stated above, after passing through the respective optical filtercavity 416, 417, 418 (in FIG. 4), each laser beam is delivered to aninput of the Sagnac resonator assembly. For example, the laser beam fromslave laser 82 is delivered to the CW input of the Sagnac resonatorassembly 54. The laser beams from slave lasers 80 and 84 are coupledtogether in optical coupler 100. The coupled laser beams from slavelasers 80 and 84 are collinear going into the CCW input of the Sagnacresonator assembly 54.

Notably, although a plurality of optical filter cavities 416, 417, 418are used in this exemplary embodiment, a single ring resonator opticalfilter cavity 716 can be used in other embodiments. In the MFLS opticssubassembly 760 in FIG. 7, the output of the slave lasers 780 and 784are mixed in the optical coupler 100. Both the output of the opticalcoupler 100 and the laser beam from the slave laser 782 are input intothe optical filter cavity 716. Since the optical filter cavity 716 is aresonator in transmission mode, the light from each slave laser 780,782, and 784 is kept at or very near the resonance frequency of theoptical filter cavity 716 to minimize throughput loss through theoptical filter cavity 716. The collinear laser beams from slave lasers780 and 784 are output from the optical filter cavity 716 to an opticalfiber coupled to the CCW input of the Sagnac resonator assembly. Thelaser beam from slate laser 782 is output to an optical fiber coupled tothe CW input of the Sagnac resonator assembly. Thus, the laser beams forthe CW input and the CCW input are physically separated as output fromthe optical filter cavity 716 as explained with respect to the exemplaryring resonator optical filter cavity 800 in FIG. 8.

The exemplary ring resonator optical filter cavity 800 includes mirrors801, 803, 805, and channels 807, 809, 811. The mixed laser beam fromslave lasers 784 and 780 enters the ring resonator cavity 800 via one ofmirrors 801, 803, 805, such as mirror 801 in this example. The laserbeam from slave laser 782 enters the optical filter cavity 800 via thesame mirror 801, in this example. However, it is to be understood thatthe laser beam from slave laser 782 enters the optical filter cavity 800via a different mirror in other embodiments.

Although, the laser beam from slave laser 782 enters the optical filtercavity 800 via the same mirror 801, the optical fiber through which thelaser beam from slave laser 782 travels is configured to deliver thelaser beam at a different angle than the angle at which the mixed laserbeam from slave lasers 780 and 784 enter the optical filter cavity 800.Thus, only portions of the laser beam from slave laser 782 exit throughmirror 803 at the proper angle to enter optical fiber 821 coupled to theCW input of the Sagnac resonator assembly. Similarly, only portions ofthe mixed laser beam from slave lasers 784 and 780 exit mirror 805 atthe proper angle to enter optical fiber 823 coupled to the CCW input ofthe Sagnac resonator assembly. Thus, the laser beams input into the CWinput and the CCW input are kept physically isolated from one another asthey exit the optical fiber cavity 800.

In some embodiments using the single optical fiber cavity 800, opticalisolators 815 and 819 are included to prevent portions of the laserbeams exiting mirror 801 to travel back down the opposite fiber in thereverse direction. Optical isolators 815 and 819 allow light to passthrough in one direction only. For example, optical isolator 819 allowsthe mixed laser beam from slave lasers 780 and 784 to pass through tothe optical filter cavity 800. However, portions of laser beam 782exiting mirror 801 are prevented by optical isolator 819 from travelingdown the reverse path to slave lasers 784 and 780.

FIG. 9 is a schematic diagram of an exemplary RTE system such as the RTEsystem 56 shown in FIG. 2. In addition to the resonance tracking circuit66, the RTE system 56 optionally includes, in some embodiments, a firstanalog-to-digital (A/D) converter 134 coupled to an input of theresonance tracking loop 925, a second A/D converter 136 coupled to aninput of the resonance tracking loops 927 and 929, a digital-to-analogconverter (DAC) 146 coupled to an output of the resonance tracking loop925, and DAC 147, 149 coupled to a corresponding output of the resonancetracking loops 927 and 929. The A/D converter 136 digitizes CCWphotodetector signals received from the resonator coupling optics 62.Similarly, the A/D converter 136 digitizes the CW photodetector signalsreceived from the resonator coupling optics 62. In this embodiment, theDAC 146, 147, 149 convert the output of the resonance tracking loops925, 927, 929 to an analog signal.

In this exemplary embodiment, the resonance tracking circuit 66 isimplemented as a field programmable gate array (FPGA). However, in otherembodiments, other programmable logic devices known to one of skill inthe art are used. The resonance tracking circuit 66 comprisesdemodulators 931, 933, 935 and resonance tracking loops 925, 927, 929.Demodulator 931 demodulates the signal of the CW direction at apredetermined demodulation frequency, f_(n), corresponding to themodulation frequency of laser beam generated by the slave laser 82.Demodulator 933 demodulates the signal of the CCW direction at apredetermined demodulation frequency, f_(m), corresponding to themodulation frequency of laser beam generated by the slave laser 84.Demodulator 935 demodulates the signal of the CCW direction at apredetermined demodulation frequency, f_(f), corresponding to themodulation frequency of laser beam generated by the slave laser 80.

The resonance tracking loop 925 locks the CW laser beam onto the CWresonance frequency, f₁, and provides a signal indicating the beatfrequency, Δf₁. Similarly, the resonance tracking loops 927 and 929 lockthe CCW laser beam onto the corresponding CCW resonance frequency, f₂ orf₃, and provide a signal indicating the corresponding beat frequenciesΔf₂ and Δf₃. The outputs of the resonance tracking loops 925, 927, and929 are provided to the MFLS 58 for controlling the frequency of theslave lasers 82, 84, and 80, respectively. In addition, the beatfrequencies Δf₁, Δf₂, and Δf₃ can be output to a processor via interfacelogic 170 for calculating the rotational rate of the gyro based on thebeat frequencies as described above.

As stated above, FIG. 10 is a block diagram of another embodiment of areference laser generator 1000. Reference laser generator 1000 includesreference resonator 1004, photodetector 1006, differential amplifier1008, servo electronics 1012, current driver 1013 and master laser 1002which function as the corresponding components in the reference lasergenerator 210. However, reference laser generator 1000 also includesoptical filter cavity 1014 and Relative Intensity Noise (RIN) servo 1016that controls out intensity fluctuations. The optical filter cavity 1014functions similar to the optical filter cavities discussed above. Inparticular, the optical filter cavity 1014 removes high-frequency jitterfrom the laser beam produced by the master laser 1002. Removing thehigh-frequency jitter from the laser beam enables the requirements oflinearity placed on the reference resonator 1004 to be alleviated. Whilestill needing linearity at the sides of the resonance curve, therequirements for linearity are not as stringent as when an opticalfilter cavity is not used. Thus, a less expensive reference resonator1004 can be used.

RIN is the noise or fluctuation in the intensity of the laser beam. Thelaser beam initially output by the master laser 1002 may have RIN. Inaddition, intensity fluctuations can be introduced into the laser afterpassing through the optical filter cavity 1014. The intensityfluctuation can be mistaken for frequency fluctuations, since thefrequency fluctuations are converted linearly to intensity fluctuationsvia the reference resonator 1004.

The RIN servo 1016 includes an optical coupler 1018, photodetector 1020,RIN servo electronics 1022 and intensity modulator 1024. The opticalcoupler 1018 couples a portion of the laser beam to the photodetector1020 which converts the intensity of the optical signal to an electricalsignal. The electrical signal is provided to the RIN servo electronics1022 which generates command signals for controlling the intensitymodulator 1024. In this example, the intensity modulator 1024 isimplemented as a variable optical attenuator. The intensity modulator1024 introduces an equal and opposite intensity fluctuation into thelaser beam to cancel the intensity fluctuation present in the laserbeam. Thus, when the laser beam enters the reference resonator 1004, theRIN has been substantially removed from the laser beam which furtherimproves the stability of the reference laser generated by the referencelaser generator 1000. It is to be understood that, in some embodiments,a RIN servo, similar to RIN servo 1016, is also used after each opticalfilter cavity in the MFLS optics subassembly discussed above.

Furthermore, an alternative embodiment of an MFLS optics subassembly1100 is shown and described in FIG. 11. In the MFLS optics subassembly1100, a reference laser generator is not included. In lieu of areference laser generator, each laser 1101, 1103, and 1105 is directlylocked to the linear portion of a resonance curve in a referenceresonator rather than via a reference laser. In particular, a lightstabilization loop comprising an optical filter cavity 1114, a RIN servo1116, a reference resonator 1104, photodetector 1106, differentialamplifier 1108, servo electronics 1112, and current driver 1113 isincluded in the optical path of the output of each laser 1101, 1103, and1105. Each of the elements functions as discussed above with respect toFIGS. 4 and 10. Furthermore, in some embodiments, a single opticalfilter 1114 is used for all of the lasers 1101, 1103, and 1105 ratherthan including the optical filter 1114 in the light stabilization loopof each laser 1101, 1103, and 1105.

Optical couplers 88 are used to couple a portion of the stabilized andfiltered laser beam to an input of the Sagnac resonator assembly. Thus,the MFLS optics subassembly 1100 also substantially removes both low andhigh frequency jitter from the laser beams produces by lasers 1101,1103, and 1105.

FIG. 12 is a flow chart depicting one embodiment of a method 1200 ofdetermining a rotational rate of a resonator gyroscope. At block 1202, aplurality of laser beams are generated. For example, in someembodiments, two laser beams are generated for input to a Sagnacresonator, whereas, in other embodiments, three laser beams aregenerated for input to a Sagnac resonator as described above. Inaddition, in some embodiments, generating a plurality of laser beamsincludes generating a reference laser beam from a master laser in areference laser generator.

At block 1204, each of the plurality of laser beams is passed through anoptical filter cavity to remove high-frequency fluctuations from each ofthe plurality of laser beams. For example, in some embodiments each ofthe plurality of laser beams is passed through a separate optical filtercavity for each of the laser beams. In other embodiments, two or more ofthe plurality of laser beams are passed through a single ring resonatoroptical filter cavity as described above with respect to FIG. 6

At block 1206, each of the plurality of laser beams is locked to afrequency of a linear portion of a resonance curve of a referenceresonator to remove low-frequency fluctuations from each of theplurality of laser beams. For example, in some embodiments, each of theplurality of laser beams is locked directly to a frequency of a linearportion of the resonance curve as described above with respect to FIG.11. In other embodiments, a reference laser is locked directly to afrequency of a linear portion of the resonance curve and the other laserbeams are locked to the reference laser as described above with respectto FIG. 4

At block 1208, one of the plurality of laser beams tuned to theresonance frequency of the first counter-propagating direction isprovided to a Sagnac resonator in the first counter-propagatingdirection (e.g. CW direction). At block 1210, at least one of theplurality of laser beams is provided to the Sagnac resonator in a secondcounter-propagating direction (e.g. CCW direction), as described above.For example, in some embodiments, two laser beams are provided to theSagnac resonator in the second counter-propagating direction. One of thetwo laser beams is tuned to a resonance frequency that is at least oneresonance mode lower than the resonance frequency of the firstcounter-propagating direction and the other laser beam is tuned to aresonance frequency that is at least one resonance mode higher than theresonance frequency of the first counter-propagating direction, asdescribed above. In other embodiments, a single laser beam is providedto the Sagnac resonator in the second counter propagating direction. Thesingle laser beam is switched between a resonance frequency that is atleast one resonance mode lower than the resonance frequency of the firstcounter-propagating direction and a resonance frequency that is at leastone resonance mode higher than the resonance frequency of thefirst-counter-propagating direction.

At block 1212, a first beat frequency based on the first resonancefrequency for the first counter-propagating direction is determined,such as through the resonance tracking electronics as described above.At block 1214, a second beat frequency based on a second resonancefrequency for the second counter-propagating direction is determined andat block 1216, a third beat frequency based on a third resonancefrequency for the second counter-propagating direction is determined, asdescribed above. The rotational rate of the resonator gyroscope is afunction of the first, second and third beat frequencies as describedabove.

FIG. 13 shows the light frequency transfer function of an exemplaryoptical filter cavity, such as the optical filter cavities describedabove. Frequency fluctuations below the filter cutoff frequency willpass through the filter unattenuated, whereas frequency fluctuationsabove the filter cutoff frequency will be attenuated. The filter cutofffrequency is related to the linewidth of the optical filter resonator. Anarrower linewidth results in a lower cutoff frequency. A lower cutofffrequency is desired to minimize the adverse impact of down convertedlaser frequency noise. However, lowering the optical filter cutofffrequency can also impose a limitation on the bandwidth of the laserstabilization servo (e.g. servo electronics 1012). If the laserstabilization bandwidth is not sufficiently large, then the servoelectronics will not adequately reduce laser frequency noise within therequired frequency band. Careful feedback loop design can alleviate thelimitation imposed by the filter cutoff frequency.

FIG. 14 shows an exemplary Bode plot of loop gain for an exemplaryfeedback loop, such as in reference laser generator 1000, that has beenoptimized to address the control bandwidth limitation imposed by anexemplary optical filter cavity. In this example, the total loop gainroll off is 20 dB per decade of frequency or less when the loop gaincrosses unity. In the laser stabilization loop the lock point would beat the side of the frequency discriminator resonator where the frequencyto intensity conversion factor is high and the non-linearity is low, asdescribed above. For a loop that involves an electronic integrator,exemplary values of the total loop gain include a roll off of 20 dB perdecade below the cutoff of the optical filter and 40 dB per decade abovethe optical filter cutoff frequency. Thus, in this example, the unitygain crossover frequency is less than the cutoff frequency of theoptical filter if a simple electronic integrator is used. By introducinga zero in integrator transfer function, the limitation imposed by theoptical filter can be removed.

FIG. 14 also shows an exemplary Bode plot of an exemplary electronicintegrator with a zero, such as servo electronics 1012. The gain of theintegrator rolls off at a rate of 20 dB per decade until reaching thezero in it's transfer function, than flattens out above the zerofrequency. This allows the overall loop gain to be increased such thatthe zero frequency occurs well below the unity crossover frequency ofthe total loop gain and the roll off of the total loop gain is 20 dB perdecade at the unity crossover frequency. Therefore by using anintegrator with a zero, a feedback loop can be made to have a greatercontrol bandwidth then the cutoff frequency of the optical filter. Thisallows the cutoff frequency of the optical filter cavity to be setsufficiently low to minimize the adverse impact of down conversion oflaser frequency noise. Even though the optical filter passivelyattenuates frequency noise at high frequencies, because it is in theloop the output from coupler 88 to the MFLS will still have frequencynoise at high frequencies. However, optical filters 416, 417, and 418,for example, mitigate this high frequency laser noise in the slavelasers.

FIG. 15 is a block diagram of an exemplary embodiment of an opticalfilter resonator 1500. Silicon optical bench (SiOB) technology isemployed in this example to make very small low cost resonators for theoptical filter resonator 1500. In this example, the resonator fiber 1502is wound on a Piezoelectric Transducer (PZT) tube 1504. This allowscontrol of the resonance frequency of the resonator 1500 via a controlvoltage. Filter resonator servo electronics 1506 maintains the resonatorresonance frequency at the laser frequency. This maximizes the opticalthroughput through the filter resonator 1500.

The optical filter resonator 1500 is operated in transmission mode,which provides the low pass filtering of the high-frequencyfluctuations. For transmission mode, substantially all of the resonatoroutput light goes through the resonator coil 1508. The optical filterresonator 1500 also includes a plurality of lenses 1510, mirrors 1512,and a photodiode 1514 on a Silicon optical bench 1518 which are used tocontrol circulation of light through the resonator fiber 1502.

FIG. 16 is a block diagram of an exemplary embodiment of a frequencydiscriminator resonator 1600. The frequency discriminator resonator 1600is operated in reflection mode, which provides a very fast response tolaser frequency fluctuations. For reflection mode, a significant portionof the resonator output light has not gone through the resonator coil1608. The frequency discriminator resonator fiber 1602 is also wound ona PZT tube 1604, which allows control of the resonance frequency. Thefrequency discriminator resonance frequency is controlled to be a fixedrelationship with the resonance frequency of the gyro Sagnac resonatordescribed above. Frequency discriminator resonator 1600 also includes,in this example, a plurality of lenses 1610, a mirror 1612, and aphotodiode 1614 on a Silicon optical bench 1618 which are used tocontrol the circulations of light through the resonator fiber 1602.

Hence, the embodiments described above enable improved performance ofthe resonator gyro due to the removal of both low-frequency andhigh-frequency jitter. For example, since resonator fiber optic gyros(RFOGs) use a resonator as the Sagnac sensing element, narrowband laserlight sources are typically required with low phase noise, and theserequirements are made more stringent when combined with standard methodsto reduce the effects of optical backscatter. Typically only verysophisticated, and expensive lasers meet these requirements. However, byusing the techniques described above, a less expensive narrow bandsingle mode semiconductor (SC) laser can be used despite the high degreeof high frequency phase noise that rectifies into intensity noise, bothat the frequency of the noise and also at subharmonics of it, whenpassed through the Sagnac resonator. Indeed, the frequency of the phasenoise in SC lasers may be too high to stabilize through conventionalelectronic frequency stabilization by locking to a reference. The highfrequency phase noise of typical inexpensive narrowband SC lasersresults in increased and undesirable uncertainty in the rotationmeasurement. However, the techniques described above substantiallyremove both low-frequency and high-frequency phase noise enabling theuse of inexpensive narrowband SC lasers. In addition, high-performancelasers also benefit from the reduction in phase noise thereby providingincreased sensitivity and stability in rotation measurements.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. For example,although the exemplary embodiments described above include three slavelasers, it is to be understood that other appropriate numbers of slavelasers can be used in other embodiments. In particular, in one exemplaryembodiment, two slave lasers are used. In such an embodiment, one slavelaser provides a laser beam to the CW input and the other slave laserprovides a laser beam to the CCW input of the Sagnac resonator assembly.Appropriate changes are also made to the RTE tracking circuit and theMFLS to accommodate 2 lasers versus 3 lasers. Therefore, it ismanifestly intended that this invention be limited only by the claimsand the equivalents thereof.

1. A resonator gyroscope comprising: a reference laser generator, thereference laser generator comprising: a master light source to produce areference light; a frequency discriminator to linearly convert frequencyfluctuations of the reference light to intensity fluctuations; aphotodetector to convert fluctuations in the intensity of an opticaloutput of the filter discriminator to an electrical voltage; adifferential amplifier to compare the output voltage of thephotodetector with a stable reference voltage; and servo electronics toprovide control signals to the master light source based on an output ofthe differential amplifier to drive the master light source such thatlow-frequency fluctuations in the reference light are removed; theresonator gyroscope further comprising: a first slave light sourcehaving a first modulation frequency to produce a first slave lighthaving a first frequency; a first phase-lock-loop (PLL) to beat thefirst slave light with the reference light and to drive the first slavelight source such that the first slave light is locked to the referencelight; a second slave light source having a second modulation frequencyto produce a second slave light having a second frequency; a secondphase-lock-loop (PLL) to beat the second slave light with the referencelight and to drive the second slave light source such that the secondslave light is locked to the reference light; a first optical filtercavity coupled to at least one of the first and second slave lightsources to filter out high-frequency fluctuations in the respectivefirst and second slave lights; a resonator coupled to said first andsecond light sources, the resonator having first and secondcounter-propagating directions and comprising an optical fiber coil,wherein the resonator circulates a first circulating light through theoptical fiber coil in the first counter-propagating direction, the firstcirculating light based on a portion of the first slave light; andcirculates a second circulating light through the optical fiber coil ina second counter-propagating direction, the second circulating lightbased on a portion of the second slave light; and resonance trackingelectronics coupled to the resonator to generate a first beat frequencybased on a first resonance frequency for the first counter-propagatingdirection, a second beat frequency based on a second resonance frequencyfor the second counter-propagating direction, and a third beat frequencybased on a third resonance frequency for the second counter-propagatingdirection; wherein the second resonance frequency is at least oneresonance mode lower than the first resonance frequency and the thirdresonance frequency is at least one resonance mode higher than the firstresonance frequency and the rotational rate of the resonator gyroscopeis a function of the first, second and third beat frequencies.
 2. Theresonator gyroscope of claim 1, wherein the reference laser generatorfurther comprises: an optical filter cavity coupled to an output of themaster laser to filter out high-frequency fluctuations in the referencelight; and a relative intensity noise (RIN) servo coupled to an outputof the optical filter cavity to control out intensity fluctuations inthe reference light.
 3. The resonator gyroscope of claim 1, wherein thefrequency discriminator comprises an optical resonator having a linearportion in its resonance curve and wherein the servo electronics lockthe master light source to a frequency in the linear portion of theresonance curve such that frequency fluctuations are converted linearlyto intensity fluctuations.
 4. The resonator gyroscope of claim 1,further comprising: a third slave light source having a third modulationfrequency to produce a third slave light having a third frequency; and athird phase-lock-loop (PLL) to beat the third slave light with thereference light and to drive the third slave light source such that thethird slave light is locked to the reference light; wherein theresonator circulates a third circulating light through the optical fibercoil in the second counter-propagating direction, the third circulatinglight based on a portion of the third slave light.
 5. The resonatorgyroscope of claim 4, further comprising: a second optical filter cavitycoupled to the second slave light source to filter out high-frequencyfluctuations in the second slave light; and a third optical filtercavity coupled to the third slave light source to filter outhigh-frequency fluctuations in the third slave light; wherein the firstoptical filter cavity is coupled to the first slave light source tofilter out high-frequency fluctuations in the first slave light.
 6. Theresonator gyroscope of claim 5, wherein each of the first, second, andthird optical filter cavities is a ring resonator optical filter cavity.7. The resonator gyroscope of claim 4, wherein the first optical filtercavity is a ring resonator optical filter cavity coupled to each of thefirst, second, and third slave light sources to filter outhigh-frequency fluctuations in each of the first, second, and thirdslave light sources.
 8. The resonator gyroscope of claim 1, furthercomprising a relative intensity noise (RIN) servo coupled to an outputof the first optical filter cavity to control out intensity fluctuationsin the light output from the first optical filter cavity.
 9. A resonatorgyroscope comprising: a plurality of light sources, each having amodulation frequency; at least one optical filter cavity coupled to anoutput of at least one of the plurality of light sources to filter outhigh-frequency fluctuations in the light from the at least one lightsource; one or more light stabilization loops, each of the one or morelight stabilization loops coupled to one of the plurality of lightsources and comprising: a resonator having a linear portion in itsresonance curve, the resonator converting frequency fluctuations of thelight from the respective one of the plurality of light sources tointensity fluctuations; a photodetector coupled to an output of theresonator to convert fluctuations in the intensity of an optical outputof the resonator to an electrical voltage; a differential amplifier tocompare the output voltage of the photodetector with a stable referencevoltage; and servo electronics to provide control signals to therespective one of the plurality of light sources to lock the modulationfrequency of the respective light source to a frequency in the linearportion of the resonance curve such that frequency fluctuations areconverted linearly to intensity fluctuations, the control signals alsodriving the respective light source such that low-frequency fluctuationsin the light from the respective light source are removed; the referenceresonator further comprising: a Sagnac resonator coupled to theplurality of light sources, the Sagnac resonator having first and secondcounter-propagating directions and comprising an optical fiber coil,wherein the Sagnac resonator circulates a first circulating lightthrough the optical fiber coil in the first counter-propagatingdirection, the first circulating light based on a portion of light fromone of the plurality of light sources received via the at least oneoptical filter cavity; and circulates a second circulating light throughthe optical fiber coil in a second counter-propagating direction, thesecond circulating light based on a portion of the light from another ofthe plurality of light sources received via the at least one opticalfilter cavity; and resonance tracking electronics coupled to the Sagnacresonator to generate a first beat frequency based on a first resonancefrequency for the first counter-propagating direction, a second beatfrequency based on a second resonance frequency for the secondcounter-propagating direction, and a third beat frequency based on athird resonance frequency for the second counter-propagating direction;wherein the second resonance frequency is at least one resonance modelower than the first resonance frequency and the third resonancefrequency is at least one resonance mode higher than the first resonancefrequency and the rotational rate of the resonator gyroscope is afunction of the first, second and third beat frequencies.
 10. Theresonator gyroscope of claim 9, wherein the plurality of light sourcescomprises a first light source having a first modulation frequency, asecond light source having a second modulation frequency, and a thirdlight source having a third modulation frequency, wherein the firstlight source frequency is tuned to the first resonance frequency via afirst phase-lock loop (PLL), the second light source frequency is tunedto the second resonance frequency via a second PLL, and the third lightsource frequency is tuned to the third resonance frequency via a thirdPLL.
 11. The resonator gyroscope of claim 10, wherein the one or morelight stabilization loops comprises: a first light stabilization loopcoupled to the first light source; a second light stabilization loopcoupled to the second light source; and a third light stabilization loopcoupled to the third light source.
 12. The resonator gyroscope of claim11, wherein the at least one optical filter cavity comprises a singlering resonator optical filter cavity coupled to each of the first,second, and third light sources to filter out high-frequencyfluctuations in each of the first, second, and third light sources. 13.The resonator gyroscope of claim 10, wherein the plurality of lightsources includes a master light source to produce a reference light,wherein the one or more light stabilization loops comprises a singlelight stabilization loop coupled to the master light source, whereinlight from each of the first light source, the second light source, andthe third light source is locked to the reference light via the firstPLL, the second PLL, and the third PLL, respectively.
 14. A method ofdetermining a rotational rate of a resonator gyroscope, the methodcomprising: generating a plurality of laser beams; passing each of theplurality of laser beams through an optical filter cavity to removehigh-frequency fluctuations from each of the plurality of laser beams;locking each of the plurality of laser beams to a frequency of a linearportion of a resonance curve of a reference resonator to removelow-frequency fluctuations from each of the plurality of laser beams;providing one of the plurality of laser beams to a Sagnac resonator in afirst counter-propagating direction; providing at least one of theplurality of lasers to the Sagnac resonator in a secondcounter-propagating direction; determining a first beat frequency basedon a first resonance frequency for the first counter-propagatingdirection; determining a second beat frequency based on a secondresonance frequency for the second counter-propagating direction, thesecond resonance frequency at least one resonance mode lower than thefirst resonance frequency; and determining a third beat frequency basedon a third resonance frequency for the second counter-propagatingdirection, the third resonance frequency at least one resonance modehigher than the first resonance frequency; wherein the rotational rateof the resonator gyroscope is a function of the first, second and thirdbeat frequencies.
 15. The method of claim 14, wherein providing at leastone of the plurality of lasers to the Sagnac resonator in the secondcounter-propagating direction comprises: providing a second laser beamtuned to the second resonance frequency to the Sagnac resonator in thesecond counter-propagating direction and providing a third laser beamtuned to the third resonance frequency to the Sagnac resonator in thesecond counter-propagating direction.
 16. The method of claim 14,wherein passing each of the plurality of laser beams through an opticalfilter cavity comprises passing each of the plurality of laser beamsthrough a separate optical filter cavity for each of the plurality oflaser beams.
 17. The method of claim 14, wherein passing each of theplurality of laser beams through an optical filter cavity comprisespassing more than one of the plurality of laser beams simultaneouslythrough the same optical filter cavity.
 18. The method of claim 14,further comprising: passing one or more of the plurality of laser beamsthrough a relative intensity noise (RIN) servo to control out intensityfluctuations in the one or more laser beams of the plurality of laserbeams.
 19. The method of claim 14, wherein locking each of the pluralityof laser beams to a frequency of a linear portion of the resonance curveof the reference resonator comprises locking a first laser beam of theplurality of laser beams directly to a frequency of a linear portion ofthe resonance curve of the reference resonator; and locking the otherlaser beams of the plurality of laser beams to the first laser beam. 20.The method of claim 14, wherein providing at least one of the pluralityof laser beam to the Sagnac resonator in the second counter-propagatingdirection includes: switching the at least one laser beam between thesecond resonance frequency and the third resonance frequency.